The present invention relates generally to electrochemical double-layer capacitors, and more particularly to structure and method of manufacture of such capacitors utilizing polymer electrolytes with increased energy and power densities, improved stability, lower leakage, lower manufacturing cost and improved form factor.
Increase in volumetric energy density, high cycle life, greater reliability and low cost are some of the most important requirement for capacitors utilized in various military and commercial applications. Conventional dielectric capacitors such as plastic film capacitors and ceramic capacitors can accumulate and deliver electric charge very rapidly, i.e. they can operate in pulse mode with pulse widths in the nanosecond (ns) scale. However, their charge storage capability is rather poor compared to electrochemical capacitors. A dielectric capacitor with planar metal plates has capacitance in the range of pico to nano farads (pF, nF, resp.) per square centimeter (cm2) (B. E. Conway, Journal of the Electrochemical Society, Volume 138, p. 1539, (1991); I. D. Raistrick, Electrochemical Capacitors, LA-UR-90-39 (January 1990); B. E. Conway, xe2x80x9cElectrochemical Supercapacitors: Scientific Fundamentals and Technological Applicationsxe2x80x9d Kluwer, Academic/Plenum Publishers (1999).
Plastic film capacitors can be tailored for very high voltages simply by adjusting the film or dielectric thickness in the capacitor. The energy density of commercial film capacitors based on polyester or polypropylene is less than 1 joule per cubic centimeter (J/cc). Impregnated film capacitors have a very narrow operating temperature range while the metallized version can operate up to 100xc2x0 C. with the exception of polyphenylene sulfide and Teflon(trademark) that can reach an operating temperature range of 200xc2x0 C.
Ceramic capacitors have an attractive form factor, high capacitance-voltage (CV) density, very good thermal withstanding, and have been widely used as miniature devices in low stress applications. Unfortunately, in power applications that require large capacitance, high voltage and excellent volumetric efficiencies, ceramic capacitors have not met expectations.
Electrolytic capacitors, as exemplified by the aluminum and tantalum electrolytics, also suffer from a number of performance limitations. The dielectric constants of the aluminum oxide and tantalum oxide dielectrics are about 10 and 28, respectively. Their breakdown voltages are at least an order of magnitude lower than polymers, however, offering little if any net advantage. Their maximum operating voltage is about 400 volts (V). Highest practical energy density achieved has been about 3 J/cc. They suffer from relatively very high leakage, very high dissipation factor (DF), hydrogen and electrolyte outgassing, reforming periodically, high equivalent series resistance (ESR) and form factor. At frequencies above 200 kilohertz (KHz), electrolytic capacitors fail from dielectric instability and poor impedance response.
Electrochemical capacitors are symmetric devices in which the electrolyte is placed between two identical electrode systems. While electrochemical capacitors can store and deliver charge in the time scale of the order of several tens of seconds, their ability to deliver charge at short times is dictated by kinetics of the surface redox (oxidation-reduction) reactions and combined resistivity of the matrix and electrolyte. Electrochemical capacitors fall into two broad categories: (1) double layer capacitors which rely solely on interfacial charge separation across the electrical double layer; and (2) pseudocapacitors which have enhanced charge storage (similar to a battery, but to a lesser extent) derived from faradaic charge transfer in parallel with the double layer. The double layer, created naturally at an electrode/electrolyte interface, has a thickness of about 10 Angstroms (A). For a high area electrode, the capacitance per unit geometric area is amplified by the roughness factor, which could approach 100,000 times. The specific capacitance is further increased in electrode systems having a substantial potential region over which a faradaic reaction (similar to a battery reaction, but to a lesser degree) takes place. Thus electrochemical capacitors, unlike their electrostatic counterparts, can accumulate substantial charge, because of the molecular level charge separation coupled with the high charge density associated with the surface redox processes on high area electrodes.
The projected energy density for electrochemical capacitors is two orders of magnitude lower than that of batteries, but power densities are several orders of magnitude higher. Energy density is much better than for conventional film capacitors but in terms of power, the electrochemical capacitors are more suitable for relatively long discharges (milliseconds (ms) to several seconds) and low to intermediate power applications. Carbon capacitors exhibit high cycle life and good stability, thus making them useful in applications such as lightweight electronic fuses, backup power sources for calculators, and surge-power delivery devices for electric vehicles. Recently, carbon capacitors have been used in small toy cars. Carbon based capacitors utilize very thick electrodes in their construction, resulting in poor particle to particle contact of the agglomerate and high ionic resistance from the electrolyte distributed in the microporous structure. The electrodes are made highly porous allowing for air and sulfuric acid to penetrate deep into the porous structure to achieve the full benefit of the surface area. Although this results in high capacitance and energy density, the ESR increases as a result of the highly porous and thick structure.
Although the pseudocapacitors utilizing valve metal oxide electrodes such as ruthenium or iridium oxide possess very high double layer capacities emanating from the intrinsic high surface areas and redox processes, leading to energy densities as high as 10 to 20 J/cc, they suffer from the same limitations as the carbon capacitors with high ESR. Ruthenium oxide has a high double layer capacity of about 150 microfarads per real square centimeter (xcexcF/real cm2). Since the intrinsic surface area of this material is very high, it is probable that the intrinsic capacitance will also be extremely high. The superior, demonstrated performance of the RuO2-based capacitor is a consequence of the high exchange current density of the RuO2/Ru2O3 reaction, although this advantage is negated by the porous nature of the RuO2 matrix used in such devices. Craig, in Canadian Patent No. 1,196,683 (1985), describes a supercapacitor based on ruthenium oxide and mixtures of ruthenium and tantalum oxides and reported capacitances as high as 2.8 F/cm2. Increase in the ESR of the capacitor is a consequence of the reduction in the exchange current density. This may be overcome if the capacitor is designed with ultra-thin electrodes and highly conductive thin film electrolytes.
Electrochemical capacitors based on RuO2 and solid polymer electrolyte have been extensively studied at Giner, Inc (MA). The use of a solid polymer electrolyte leads to a leak-free system that contains no corrosive liquid electrolyte. This concept was based on the use of a hydrated ionomer membrane such as DuPont""s Nafion(trademark). The composite structure ensured a continuous proton-conducting ionomer linkage throughout a single cell, thus facilitating proton transport from one electrode to the other. The performance of this capacitor containing only hydrated water dropped off abruptly below the freezing point of water and in addition, the ESR was fairly high at about 0.3 ohm-cm2. Subsequent use of sulfuric acid improved the proton conductivity within the particulate by accessing pores down to 100 A diameter.
This study was interesting and demonstrated that high proton conductivity and materials based on very high exchange current densities is effectively required for lowering the ESR. However, the problem with using Nafion(trademark) type membranes is that they are fairly thick, resulting in high internal resistance and also very weak polymers. Swelling of the membrane by the sulfuric acid decreases its strength and conductivity even further. A polymer electrolyte that can be mechanically stable and designed in very thin film and highly conducting form would be desirable for reducing the internal resistance.
In order to obtain high energy content per unit weight and volume, it is necessary to utilize electrochemically active materials of significantly higher energy content than in present commercial capacitors. The best possibilities lie in a capacitor that incorporate materials based on high surface area activated carbon or valve metal oxides such as RuO2. Furthermore, in order to access the entire porous structure of RuO2 efficiently and achieve high capacitance (hence, high energy) and high power at low ESR, the electrode needs to be designed in very thin film form. Thinner electrodes are more feasible with pseudocapacitors than with double layer capacitors due to the greater capacitance density of the former.
Experience has shown that higher cyclability, higher power, lower internal resistance and greater capacity utilization is favored by designs that incorporate very thin electrode and electrolyte structures. Ultra-thin electrode and electrolyte will overcome kinetic constraints on the specific power, cycling efficiency and capacity utilization. The thinner the electrode, the shorter is the time needed to access regions of the structure farthest from the macroscopic electrode/electrolyte interface, thus opening up the possibility of constructing the more compact bipolar stacks necessary for high voltage, pulse power applications. In addition, improving the capacity of the electrode is a very important feature for devices that requires very long discharge times such as, for example, in electric vehicles or in cellular telephones.
The energy density of an electrochemical capacitor can further be increased if very thin inactive substrate materials such as metallized plastic current collectors are used. The use of such substrates will also result in low-cost devices. Electrochemical capacitors are lower voltage devices; aqueous based are 1 V/cell and non-aqueous based are about 3 to 4 V/cell. Connection of devices in series to obtain higher voltages results in a decrease in capacitance as well as an increase in ESR, according to the number of units in series. One of the advantages of using liquid organic electrolytes is the theoretical wider electrochemical window. An immediate consequence is an increase of the energy density (Energy={fraction (1/2)}(CV2), where C is static capacitance) and the power (Power=V2/R) densities.
Accordingly, it is a principal object of the present invention to provide a base polymer material for a polymer electrolyte that is dimensionally stable in the liquid solvents, aqueous or non-aqueous, presently used in electrochemical capacitors, and that is highly stable with temperature.
Another object of the invention is to provide a base polymer material for a polymer electrolyte that exhibits little or no swelling characteristics when in contact with liquid solvents, compared to prior art membranes.
Another object of the invention is to provide a base polymer material that is predominantly amorphous in nature.
Still another object is to provide a base polymer material for a polymer electrolyte that is mechanically stronger than prior art membranes when in contact with liquid solvents.
Still another object is to provide a polymer electrolyte with high ionic conductivity.
Yet another object of the invention is to provide polymer electrolyte compositions which are more conductive at lower levels of liquid solvents than prior art polymer electrolyte-solvent compositions.
Another object of the invention is to provide polymer electrolyte compositions in ultra-thin film form.
Another object of the invention is to provide polymer electrolyte compositions with a wide temperature range of operation.
Yet another object of the invention is to provide polymer electrolyte compositions with ionomer or ionically conductive backbone to further facilitate the conduction process.
A further object of the invention is to provide polymer electrolyte compositions in which the solvent is immobilized in the polymer, to allow electrochemical capacitors constructed from such compositions to be used in any orientation.
Another object of the invention is to provide polymer electrolyte compositions that can be manufactured in very thin film form, provide low resistance and excellent flexibility.
Yet another object of the invention is to provide electrochemical capacitor electrodes that are ultra-thin and conductive.
Still another object of the invention is to provide methods of manufacturing such ultra-thin and conductive electrodes.
Another object of the invention is to provide electrochemical capacitor electrodes with high capacity.
Still another important object of the invention is to provide an electrochemical capacitor with polymer electrolyte compositions and ultra-thin electrodes described in the preceding enumerated objects.
A further object is to provide such electrochemical capacitors with ultra-thin current collectors such as very thin metallic elements or metallized polymer substrates for improved energy density, power density, higher capacity utilization, higher cycle life, greater charge-discharge efficiencies, lower ESR, greater safety, and greater reliability, and which can be produced at high speed, lower cost, and with improved form factors.
Another object of the invention is to coat the thin substrate with very thin active anode and cathode material.
A related object is to laminate the anode and cathode elements on both sides of the metallized polymer substrate material so as to yield a highly flexible electrode.
The electrolyte of the present invention is preferably a proton conductor or is very conductive, is very flexible and somewhat dry, is of low cost, and in some preferred embodiments of the invention is constructed in very thin film format. Polymer electrolytes of this design can be combined with various similar electrode materials such as carbon, materials from the valve metal oxides to provide electrochemical capacitors having high specific energy (Wh/kg) (gravimetric) and energy density (Wh/l) (volumetric), high cycle life, low ESR, low leakage, and which provide improved safety.
One embodiment of a solid base polymer material of a polymer electrolyte of an electrochemical capacitor according to the invention is a thin film polymer selected from a group consisting of polyester (PET), polypropylene (PP), polyethylene napthalate (PEN), polycarbonate (PC), polyphenylene sulfide (PPS), polyvinylidene-fluoride (PVDF), and polytetrafluoroethylene (PTFE), or a combination of two or more thereof. The specific polymer and its concentration in the polymer electrolyte are selected to tailor at least one desired property of the polymer electrolyte. The base polymer material may include a perfluorocarbon-sulfonated ionomer electrolyte such as Nafion(trademark), 2-acrylamido-2-methyl propane sulfonate (or AMPS), or the Dow membrane XUS13204.10 or other ionomer materials based on different blends of fluoropolymers, including poly(chlorotrifluoroethylene), poly(ethylene-chlorotrifluoroethylene), poly(fluorinated ethylene-propylene), polytetrafluoroethylene, hexafluoropropene and polyvinylidene-fluoride (PVDF) and mixtures of these ionomers. Such materials have a perfluorocarbon polymer backbone to which sulfonic acid sites are permanently anchored. Or the base polymer material may include an ionically conducting polymer such as an acrylate, polyethylene oxide (PEO), polypropylene oxide (PPO), poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polymethyl-acrylonitrile (PMAN), or other suitable ionically conductive polymer or a combination of ionically conductive polymers, and so forth.
In another embodiment, an electrolyte-retaining base polymer material for an electrochemical capacitor is a polymer thin film cast from a solution of the base polymer such as PVDF and acrylate monomer/oligomer radiation cured after which a solvent in which those constituents were dissolved has substantially evaporated. A liquid or semi-liquid electrolyte solution containing a salt for ionic conduction such as a quaternary phosphonium (R4P+) salt, or a quarternary ammonium salt (R4N+), or a metal salt such as sodium, lithium, potassium, magnesium, or calcium salt, more preferably lithium, is absorbed within the thin film. R in this case is an alkyl group while the anion of the salt may be chosen from hexafluorophosphate (PF6xe2x88x92), perchlorate (ClO4xe2x88x92), tetrafluoroborate (BF4xe2x88x92), hexafluoroarsenate (AsF6xe2x88x92), tetrachloro-aluminate (AlC14xe2x88x92), trifluoromethanesulfonate (CF3SO3xe2x88x92), methide (C(SO2CF3)3xe2x88x92 and bis(tri-fluoromethane sulfonyl) imide (N(CF3SO2)2xe2x88x92). In certain embodiments, the salt is a plasticizer salt such as lithium imide or methide.
The liquid electrolyte may be chosen from a wide variety of solvents, including aqueous based sulfuric acid, or a non-aqueous based chosen from ethylene carbonate, propylene carbonate, dimethoxy methane, dimethoxy ethane, tetrahydrofuran, dimethoxy carbonate, diethyl carbonate, acetonitrile, or mixtures of such liquids or any other suitable organic solvents.
In a process of manufacture of this embodiment, the PVDF and acrylate monomer/oligomer are dissolved in a hydrocarbon solvent such as N-Methyl Pyrrolidone (NMP) to form a polymer solution, which is then cast in a thin film, in part by evaporation of the solvent. The film is then soaked in an appropriate liquid electrolyte solution containing an appropriate salt, for absorption of the electrolyte within the film, and the acrylate monomer/oligomer is cured by subjection to electron beam or ultraviolet radiation.
A dimensionally stable, highly resilient embodiment of a polymer solid-solution blend film for an electrochemical capacitor, the film being capable of electrolyte retention without appreciable swelling, is produced by a method in which PVDF and AMPS are mixed homogeneously to form a polymer blend thereof. A very high surface area inorganic fillerxe2x80x94either fumed silica or aluminaxe2x80x94having an average particle size  less than 0.05 micron (xcexcm) in diameter and a surface area of at least about 100 m2/g is then dispersed with a concentration in a range from about 0.1% to about 30% by weight into the copolymer blend to enhance the porosity and mechanical stability of the thin film into which the copolymer blend with inorganic filler is cast. Finally, the resultant film is soaked in a liquid solvent electrolyte for absorption and retention in the film. Preferably, the film is soaked in an aqueous based solvent such as sulfuric acid or a liquid organic electrolyte solvent, each of the solvent containing a salt for ionic conduction. The liquid electrolyte is immobilized in the AMPS/PVDF polymer to allow molecules of the liquid polymer (AMPS) to trap molecules of the electrolyte into pores of the film. Preferably, the liquid polymer is cross-linkable based on AMPS, or other suitable materials such as acrylates and PEO-based materials, and radiation curing is performed to cross-link the liquid polymer for trapping of molecules. Alternatively, some immobilization of the liquid organic solvent electrolyte may be achieved by using a non-ionizable liquid polymer. Dispersion of the inorganic filler into the polymer blend is performed during blending of the PVDF and AMPS.
Also provided by the present invention is an anode and cathode consisting of a first group material(s) possessing properties of high exchange current density, intrinsically high surface area, and high charge capacity in combination with a second group material(s) that essentially has an exceptionally high redox capacity. The first group of materials comprise activated carbon or valve metal oxides. Such materials consist of oxides of titanium, zirconium, hafnium, niobium, tantalum, molybdenum, tungsten, ruthenium, iridium, platinum, palladium, osmium, gold, and rhenium. The second group of materials are selected from a wide range of oxides, sulfides and selenides, or any other group well known in the prior art that are used in lithium batteries, e.g. MnO2, LiMn2O4, LixMnO2, MoS2, MoS3, MoV2O8, CoO2, Lix, CoO2, V6O13, V2O5, V3O8, VO2, V2S5, TiS2, NbSe3, Cr2O5, Cr3O8, WO3, LixNiO2, LixNiyCOzO2, LixNiyMnzO2, LixCoyMnzO2, lithium doped electronically conducting polymers such as polypyrrole, polyaniline, polyacetylene, and so forth. The first group of materials may be single oxides or multiple oxides. The second group of materials may consist of one compound or a mixture of compounds. The composition of the first group materials is 1 to 99% while the balance is from the second group of materials.
A method of producing an embodiment of the invention includes physically mixing the activated carbon or valve metal oxide or oxides with the battery active material to enhance the discharge time of the capacitor.
A method of producing another embodiment of the invention includes blending electrode active materials with polymer electrolytes of the above compositions.
A method of producing yet another embodiment of the invention is to fabricate ultra-thin film electrodes of these material in a thickness in a range from 1 xcexcm to more than 100 xcexcm.
Any of these polymer electrolyte films and electrodes may be used to form an electrochemical cell, particularly an electrochemical capacitor, by tightly sandwiching the film between thin, flexible active anode and active cathode layers.
For example, one embodiment of a thin film electrochemical capacitor is formed from a resilient flexible polymeric electrolyte thin film that includes a base polymer with inorganic filler dispersed therein to increase surface area and porosity of the film, impregnated with a semi-liquid or even dry solution of liquid polymer, organic solvent electrolyte and a salt; and a pair of spaced-apart flexible thin film electrodes, each including a polymer substrate having an adherent electrically conductive layer of the above mentioned metallic material thereon, the polymer electrolyte film being tightly sandwiched between the pair of thin film electrodes. The polymer substrate of each of the anode and cathode is preferably selected from a group of polymers including PET, PP, PPS, PEN, PVDF and PE, and each polymer substrate is metallized to form the conductive layer thereon. The ultra thin film metallized polymer substrate has a thickness in a range from about 0.5 micron to about 50 microns, thereby rendering it very flexible for ease of coating and handling, to avoid kinking and deformation thereof, during manufacture of electrochemical capacitors.
The electrochemical capacitor polymer substrate may comprise a layer of polymer material, and a low resistance metallization layer having a conductivity in a range from about 0.01 ohm per square to about 1 ohm per square overlying and adhered to a side of the polymer material. Preferably, the layer of polymer material has a non-metallized margin with a width in the range from about one mm to about three mm. Preferably, also, a low resistance metallization layer having a conductivity in the aforementioned range overlies and is adhered to each side of the polymer material, and both sides of the layer of polymer material have such a non-metallized margin present at the same edge of the layer of polymer material.
According to another aspect of the invention, an electrochemical capacitor electrode comprises an ultra thin film metal substrate for at least one of a cathode substrate and an anode substrate of an electrochemical capacitor, the ultra thin film metal substrate having a thickness in a range from about one micron to about 10 microns and may comprise one of the following metallic materials chosen from aluminum, copper, nickel, titanium, stainless steel, or an alloy such as inconel or any other suitable stable metallic material.
According to another aspect of the invention, the selected metal or alloy is etched either physically or chemically to increase the substrate intrinsic surface area.
According to yet another aspect of the invention, a method of fabricating a thin film electrochemical capacitor includes incorporating an ultra thin film metallized polymer substrate in the capacitor during fabrication thereof, wherein the polymer layer in the substrate has a thickness in a range from about 0.5 micron to about 50 microns, in conjunction with very thin film capacitor electrode/electrolyte structures having thickness less than 5 microns, respectively, wherein the thickness of the metallization layer on the polymer layer is selected according to desired conductivity thereof.
The invention also provides novel methods of coating an ultra thin film metallized polymer substrate for a thin film electrochemical capacitor with very thin film active anode material and active cathode material. One method comprises steps of milling each of the anode material and the cathode material in a separate solvent to reduce the particle size of the respective material, injecting respective ones of the materials directly onto the substrate at opposite sides thereof, and subsequently drawing each of the materials at opposite sides of the substrate into a thin film of desired thickness using wire wound rods or Mayer rods of different wire diameters to control wet slurry thickness. The substrate is coated on one side of the metallized polymer substrate, rather than both sides. Coating an anode on one side and a cathode on the other side would only apply to a bipolar electrode.
Another coating method includes incorporating each of the materials into its own aerosol mix, spraying atomized aerosol of each material directly on respective opposite sides of the film substrate while moving said substrate past the points of aerosol spray at high speed, and curing the sprayed material either by drying or radiation. Yet another coating method comprises evaporating the respective electrode material directly onto respective opposite sides of the substrate.
Also according to the invention, a method of fabricating a thin film electrochemical capacitor involves laminating anode and cathode elements on respective opposite sides of a double-metallized polymer substrate, whereby to yield a highly flexible electrode structure for the capacitor. Non-metallized margins are provided on each of the anode and cathode elements on the opposite sides of the metallized polymer substrate, and metal is sprayed on opposite ends of the laminated metallized polymer substrate for terminations thereto. These techniques enable the provision of a ratio of substrate thickness to active electrode thickness less than about 0.5.
Also according to the invention, a method of fabricating a thin film bipolar element is provided involving laminating anode and cathode active elements on respective opposite sides of a double-metallized of the same polymer substrate which has been impregnated with an electronically conductive material within the polymer substrate such as carbon black or metallic elements (inert to the active electrode), whereby to yield a highly flexible and strong electrode structure for the capacitor.
Also according to the invention, a method of fabricating a thin film bipolar element is provided as described immediately above, but the double-metallized and conductive polymer substrate is replaced by a very thin film metallic substrate.
Also, according to the invention is provided, a method of forming a bipolar element is provided also as described above, but with a coating of polymer electrolyte on each side of the bipolar electrode.